What would be these altitudes, and when (as in what year)?
Do you get the difference in how a geared supercharger and a turbocharger work?
For the former, the engine speed determines the impeller speed and that is a fixed ratio. Thus, superchargers work with decreasing efficiency as they move from their best altitude and engine speed. This is why two, and even three, stage superchargers were being put on aircraft during WW 2. The Allison V 1710 had a single speed supercharger and that ran out of steam, so to speak, at about 15,000 feet.
When you go to a turbocharger, the exhaust gas pressure driving the turbine can be varied with manifold pressure and altitude to give a constant input pressure to the engine. It's as easy as having a waste gate to dump excess exhaust pressure to maintain a constant turbo boost on for the engine. They are far less dependent on altitude as a variable for performance.
Turbochargers are typically more fuel efficient. Turbochargers are driven by the engine’s exhaust gases, which is considered a type of waste energy. Rather than allowing this energy to go to waste, it’s used to turn the turbocharger and boost the engine’s power and performance.
Turbochargers can also handle higher altitudes. Superchargers have a somewhat limited ceiling. At high altitudes, they become very inefficient. Turbochargers, though, can handle high altitudes while maintaining their efficient properties.
https://monroeaerospace.com/blog/aircraft-superchargers-vs-turbochargers-whats-the-difference/
https://www.flyhpa.com/2021/04/turbocharging-vs-supercharging/
https://www.flight-study.com/2021/02/superchargers-and-turbosuperchargers.html
Here are the speed curves for a P-38 (turbocharged)
Notice how they are a smooth curve to a critical altitude where performance begins to fall off because the turbocharger can only supply so much compression as the air thins (wastegate fully shut).
Here are the ones for an FW 190 and Ta 152
Note the jagged lines. These mark the points where the supercharger changes gears. As you get away from those points, performance drops off markedly.
This is why a turbocharger, even for its drawbacks like turbo lag, is superior to a geared supercharger.
